Rail having excellent fatigue crack propagation resistance characteristics, and method of producing same

ABSTRACT

A rail has excellent fatigue crack propagation resistance characteristics, wherein the rail has a component composition including C: 0.80 to 1.30 mass %, Si: 0.10 to 1.20 mass %, Mn: 0.20 to 1.80 mass %, P: not more than 0.035 mass %, S: 0.0005 to 0.012 mass %, Cr: 0.20 to 2.50 mass % and the remainder being Fe and inevitable impurities and satisfying CP represented by equation (1) being not more than 2500:CP=X/RA  (1) andX={(10×[% C])+([% Si]/12)+([% Mn]/24)+([% Cr]/21)}5  (2),where [% Y] is content of an element Y (mass %), and RA is a prior austenite grain size (μm).

TECHNICAL FIELD

This disclosure relates to a rail and a method of producing the same and, more particularly, to a rail having an improved fatigue crack propagation resistance characteristics and a method of advantageously producing the rail.

BACKGROUND

In a high-axle load railway mainly used for transporting ores and the like, a load applied to an axle shaft of a freight car is much higher than that applied to a passenger carriage, causing a severer usage environment of a rail. The rail used under such an environment has used steel mainly having a pearlite microstructure with an emphasis on wear resistance. In recent years, however, with an increase in the load weight of a freight car to enhance transportation efficiency by rail, further improvement of the wear resistance and fatigue damage resistance of a rail has been demanded. The high-axle load railway means a railway in which a load capacity per one freight car in a train or goods train is large (for example, the load capacity is not less than about 150 tons).

In this regard, various studies have been made to further improve the wear resistance. For example, the C content is increased to more than 0.85 mass % but not more than 1.20 mass % in JP-A-H8-109439 and JP-A-H8-144016. In JP-A-H8-246100 and JP-A-H8-246101, the C content is made to more than 0.85 mass % but not more than 1.20 mass % and the head portion of the rail is subjected to a heat treatment. In those techniques, the C content is increased to increase the cementite fraction, thus improving the wear resistance.

Rolling stress by a wheel and slippage by centrifugal force is applied to a rail laid in the curved section of the high-axle load railway, causing severer wearing of the rail and fatigue damage resulting from the slippage. JP-A-2002-69585 proposes a technique to suppress formation of pro-eutectoid cementite by adding Al and Si to improve fatigue damage resistance.

Also, JP-A-2010-185106 proposes a technique of controlling a lamellar spacing of pearlite in a proper range to decrease the fatigue crack propagation rate.

In the techniques disclosed in JP '439, JP '016, JP '100 and JP '101, when simply controlling the C content to more than 0.85 mass % but not more than 1.20 mass %, the pro-eutectoid cementite is formed depending on heat treatment conditions, also causing an increase in the amount of cementite layer having a brittle pearlite lamellar structure so that the improvement of the fatigue damage resistance cannot be expected. The technique disclosed in JP '585, in which an oxide as a starting point of fatigue damage is formed by adding Al, has difficulty in suppressing the fatigue cracking. In the technique disclosed in JP '106, the pro-eutectoid cementite may be formed depending on the combination between the ingredients and production conditions, and consequently, fatigue crack propagation rate is increased so that it cannot be said that the material control is sufficient.

It could therefore be helpful to provide a rail having excellent fatigue damage resistance, particularly fatigue crack propagation resistance characteristics and a preferable method of producing the same.

SUMMARY

We prepared rails with varying contents of C, Si, Mn, and Cr and carefully studied each structure and fatigue crack propagation resistance characteristic of the rails and thus derived an ingredient parameter X corresponding to a pro-eutectoid cementite amount and a parameter CP from a prior austenite grain size R_(A). We have also found, by controlling the parameter CP within a certain range, excellent fatigue crack propagation resistance characteristics can be obtained, even if there is a large amount of pro-eutectoid cementite present.

We thus provide a rail having excellent fatigue crack propagation resistance characteristics, characterized by having a component composition comprising C: 0.80 to 1.30 mass %, Si: 0.10 to 1.20 mass %, Mn: 0.20 to 1.80 mass %, P: not more than 0.035 mass %, S: 0.0005 to 0.012 mass %, Cr: 0.20 to 2.50 mass % and the remainder being Fe and inevitable impurities and satisfying CP represented by equation (1) being not more than 2500:

CP=X/R _(A)  (1) and

X={(10×[% C])+([% Si]/12)+([% Mn]/24)+([% Cr]/21)}⁵  (2)

wherein [% Y] is the content of an element Y (mass %), and R_(A) is a prior austenite grain size (μm).

The rail having excellent fatigue crack propagation resistance characteristics more preferable has:

-   -   a. the component composition further contains at least one         selected from V: not more than 0.30 mass %, Cu: not more than         1.0 mass %, Ni: not more than 1.0 mass %, Nb: not more than 0.05         mass %, and Mo: not more than 2.0 mass %; and     -   b. the component composition further contains at least one         selected from Al: not more than 0.07 mass %, W: not more than         1.0 mass %, B: not more than 0.005 mass %, Ti: not more than         0.05 mass %, and Sb: not more than 0.05 mass %.

A method of producing a rail having excellent fatigue crack propagation resistance characteristics is characterized by heating a raw steel material having any one of the aforementioned component compositions to not higher than 1350° C. and hot rolling the material such that the finish temperature is not lower than 900° C.

The method of producing a rail having excellent fatigue crack propagation resistance characteristics preferably, after the hot rolling, conducts accelerated cooling at a cooling rate of 0.4 to 3° C./s from 900° C. to 750° C. and at a cooling rate of 1 to 10° C./s from 750° C. to a cooling stop temperature of 400 to 600° C.

Our rail and the method of producing the rail can thus stably produce a rail with fatigue damage resistance that has excellent fatigue crack propagation resistance characteristics. Such a rail contributes to increasing the service life of a rail for a high axle load railway and preventing a railway accident, bringing about a beneficial effect in the industry.

Moreover, the fatigue damage resistance can be improved by properly controlling the heat treating conditions after the hot rolling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the influence of pro-eutectoid cementite on fatigue crack propagation rate. FIG. 1(a) shows where a prior austenite grain size is approximately equal to a plastic zone size, and FIG. 1(b) shows where a prior austenite grain size is larger than a plastic zone size.

FIG. 2 is a view showing a position where a test specimen for observing a prior austenite grain size was taken out.

FIG. 3 is a view showing a position where a test specimen for fatigue crack propagation was taken out.

FIG. 4 is a view showing the shape of a test specimen used in the fatigue crack propagation test, where FIG. 4(a) is a front view, FIG. 4(b) is a side view, and FIG. 4(c) is an enlarged front view of a notch portion.

FIG. 5 is a view illustrating the shape of a test specimen used in a fatigue damage resistance test, where FIG. 5(a) is a side view, and FIG. 5(b) is a front view.

FIG. 6 is a view showing a position where a test specimen for fatigue damage resistance test was taken out.

REFERENCE SIGNS LIST

-   -   1 rail head portion     -   11 test specimen taking portion for observing prior austenite         grain size     -   12 gauge corner (GC) portion     -   13 head portion     -   14 fatigue damage resistance specimen collection area     -   15 a test specimen for fatigue crack propagation     -   16 notch portion     -   17 Nishihara-type wear specimen     -   18 wheel specimen     -   21 prior austenite grain     -   22 plastic zone     -   23 fatigue crack     -   24 pro-eutectoid cementite     -   25 cleavage fracture     -   26 increase in fatigue crack propagation rate     -   27 decrease in fatigue crack propagation rate     -   R_(A) prior austenite grain size     -   R_(P) plastic zone size

DETAILED DESCRIPTION

One example will be concretely described below. First, the reason for limiting the component composition of the steel as a raw material of the rail into the above range will be described. “%” means “mass %” unless otherwise noted.

C: 0.80 to 1.30%

C is an essential element to secure the strength of pearlite microstructure, i.e., fatigue damage resistance. When the C content is less than 0.80%, it is difficult to obtain excellent fatigue crack propagation resistance characteristics. When it exceeds 1.30%, a large amount of a pro-eutectoid cementite is formed in an austenite grain boundary during the cooling after hot rolling, bringing about an increase in a fatigue crack propagation rate. Although the pro-eutectoid cementite is present even when the C content is not more than 1.30%, the influence thereof can be avoided by controlling the prior austenite grain size, based on a relation expression mentioned below. Therefore, the C content should be 0.80 to 1.30%. The upper limit of the C content is preferably 1.00%, more preferably 0.90%.

Si: 0.10 to 1.20%

In addition to the effect as a deoxidizer, Si contributes to the reduction of fatigue crack propagation rate by increasing the pearlite equilibrium transformation temperature and reducing the lamellar spacing. Thus, the Si content needs to be not less than 0.10%. However, when it exceeds 1.20%, the weldability is deteriorated due to the high bonding force of Si with oxygen. Further, Si acts to move the eutectoid precipitation point to the low C content side so that an excessive addition of Si contributes to the formation of pro-eutectoid cementite and increases the fatigue crack propagation rate. Therefore, the Si content should be 0.10 to 1.20%. The lower limit of the Si content is preferably 0.20%, and the upper limit of the Si content is preferably 0.80%, more preferably 0.60%.

Mn: 0.20 to 1.80%

Mn contributes to the reduction of the fatigue crack propagation rate by lowering the pearlite transformation temperature and increasing the lamellar spacing. However, an Mn content of less than 0.20% does not have a sufficient effect. On the other hand, when the Mn content exceeds 1.80%, the martensitic microstructure is likely to be formed, which causes hardening and embrittlement during heat treatment and welding of the rail, leading to material deterioration. Furthermore, as Mn has the effect of moving the eutectoid precipitation point towards the low C content side, the excessive addition contributes to the formation of pro-eutectoid cementite and increases the fatigue crack propagation rate. Therefore, the Mn content should be 0.20 to 1.80%. The lower limit of the Mn content is preferably 0.30%, while the upper limit of the Mn content is preferably 1.00%, more preferably 0.60%.

P: Not More than 0.035%

The P content exceeding 0.035% deteriorates ductility. Therefore, the P content should be not more than 0.035%, preferably not more than 0.020%. On the other hand, the lower limit of the P content is not particularly limited and may be 0%; more than 0% is usual industrially. Moreover, reducing the P content excessively causes an increase in the refining cost. Therefore, the P content is preferable to be not less than 0.001% from the viewpoint of economic efficiency.

S: 0.0005 to 0.012%

S is present in steel mainly in the form of A-based inclusion (subjected to viscous deformation by working). The S content exceeding 0.012% remarkably increases the amount of the inclusion and, at the same time, forms coarse inclusion, deteriorating the cleanness of the steel material. Meanwhile, less than 0.0005% S increases the refining cost. Therefore, the S content should be 0.0005 to 0.012%. The upper limit of the S content is preferably 0.010%, more preferably 0.008%.

Cr: 0.20 to 2.50%

Cr increases the pearlite equilibrium transformation temperature to reduce the lamellar spacing, contributing to decreasing the fatigue crack propagation rate. However, less than 0.20% Cr cannot suppress the progress of the fatigue crack sufficiently, while the Cr content exceeding 2.50% increases the hardenability of steel, thus often producing martensite. In the production where martensite is not produced, the pro-eutectoid cementite is produced in the prior austenite grain boundary, and as a result, the fatigue crack propagation rate is increased. Therefore, the Cr content should be 0.20 to 2.50%. The lower limit of the Cr content is preferably 0.40%, more preferably 0.50%, while the upper limit of the Cr content is preferably 1.50%, more preferably 1.00%.

It is insufficient that each element only satisfies the above range. It is also important to control the CP value represented by equation (1) derived from ingredient parameter X corresponding to a pro-eutectoid cementite amount shown in equation (2) and prior austenite grain size R_(A) to not more than 2500:

CP=X/R _(A)  (1)

X={(10×[% C])+([% Si]/12)+([% Mn]/24)+([% Cr]/21)}⁵  (2)

wherein [% Y] is a content of element Y (mass %), and R_(A) is a prior austenite grain size (μm).

We examined the cause of the increased fatigue crack propagation rate due to the presence of pro-eutectoid cementite. As a result, we found that the preceding brittle fracture of pro-eutectoid cementite 24 at the tip of the fatigue crack 23 is the cause of the increase 26 in the fatigue crack propagation rate, as shown in the schematic view of FIG. 1(a). Furthermore, we found that the progress of brittle crack can be suppressed by adjusting the prior austenite grain size to be a formation site of the structure in correspondence to the amount of the pro-eutectoid cementite formed to thus decrease an encountering frequency of the pro-eutectoid cementite with a plastic zone 22 formed in the tip of the fatigue crack. Specifically, even when a large amount of the pro-eutectoid cementite is present as shown in FIG. 1(b), the CP value can be controlled to not more than 2500 by sufficiently coarsening the prior austenite grain size 21 larger than the size of the plastic zone 22 at the crack tip. Thus, the effect of suppressing the fatigue crack propagation rate can be obtained stably. Moreover, the CP value is preferable to be not more than 2000.

The component composition used in the rail may arbitrarily contain any one selected from at least one selected from following Group A, at least one selected from following Group B, and both thereof, in addition to the aforementioned ingredients:

-   -   Group A: V: not more than 0.30%, Cu: not more than 1.0%, Ni: not         more than 1.0%, Nb: not more than 0.05%, and Mo: not more than         2.0%     -   Group B: Al: not more than 0.07%, W: not more than 1.0%, B: not         more than 0.005%, Ti: not more than 0.05%, and Sb: not more than         0.05%.

The reason for specifying the contents of the elements belonging to Groups A and B will be described below.

V: Not More than 0.30%

V forms carbonitride in steel, which is dispersed and precipitated into the base to improve the wear resistance of steel. However, when the content exceeds 0.30%, the workability of steel is deteriorated to increase the production cost. The V content exceeding 0.30% also increases the alloying cost, causing an increase in the production cost for a high internal hardness type rail. Therefore, V is preferable to be contained up to 0.30% as an upper limit. Moreover, V is preferable to be contained by not less than 0.001% to develop the above effect of improving the wear resistance. The upper limit of the V content is more preferably 0.15%.

Cu: Not More than 1.0%

Cu is an element capable of further increasing the strength of steel by solid-solution strengthening like Cr. When the content exceeds 1.0%, however, a Cu-induced crack is easily caused. Therefore, when the component composition includes Cu, the content should be not more than 1.0%. The lower limit and upper limit of the Cu content should be 0.005% and 0.5%, respectively.

Ni: Not More than 1.0%

Ni is an element capable of increasing the strength of steel without deteriorating its ductility. Also, the Cu-induced crack can be suppressed by composite addition with Cu so that it is desirable to contain Ni when Cu is included in the component composition. When the Ni content exceeds 1.0%, however, the hardenability of steel is further improved, increasing the production amount of martensite and bainite, which often causes deterioration of the wear resistance and the fatigue damage resistance. Therefore, when Ni is included, the Ni content is preferably not more than 1.0%. Moreover, the lower limit of the Ni content is more preferably 0.005%, while the upper limit of the Ni content is more preferably 0.5%.

Nb: Not More than 0.05%

Nb bonds to C in steel during and after the hot rolling to shape the rail to form precipitates as a carbide, which acts effectively to fine the size of pearlite colony. As a result, Nb largely improves the wear resistance, fatigue damage resistance, and ductility and largely contributes to the prolonged service life of the high internal hardness type rail. When the Nb content exceeds 0.05%, the effect of improving the wear resistance and fatigue damage resistance is saturated, failing to be commensurate with the increase of the content. Therefore, Nb may be contained by up to 0.05%. When the Nb content is less than 0.001%, the effect of prolonging the service life of the rail is hard to be obtained. When Nb is included, therefore, the Nb content is preferably not less than 0.001%. Moreover, the upper limit of the Nb content is more preferably 0.03%.

Mo: Not More than 2.0%

Mo is an element capable of further increasing the strength of steel by solid-solution strengthening. Mo also moves the eutectoid precipitation point toward the high C content side to suppress the formation of the pro-eutectoid cementite. However, when it exceeds 2.0%, the amount of bainite produced in steel increases to thus deteriorate the wear resistance. Therefore, when Mo is included in the component composition of the rail, the Mo content is preferably not more than 2.0%. Moreover, the lower limit of the Mo content is more preferably 0.005%, while the upper limit of the Mo content is more preferably 1.0%.

Al: Not More than 0.07%

Al can be added as a deoxidizing agent. However, when the Al content exceeds 0.07%, a large amount of an oxide-based inclusion is produced in steel due to a high bonding force of Al with oxygen, resulting in deterioration in the ductility of steel. Therefore, the Al content is preferably not more than 0.07%. On the other hand, the lower limit of the Al content is not particularly limited but is preferable to be not less than 0.001% for deoxidization. Moreover, the upper limit of the Al content is more preferably 0.03%.

W: Not More than 1.0%

W forms precipitates as carbide during and after hot rolling to shape into a rail form and improves the strength and ductility of the rail by precipitation strengthening. When the W content exceeds 1.0%, however, martensite is produced in steel to deteriorate the ductility. Therefore, when W is added, the W content is preferably not more than 1.0%. Although the lower limit of the W content is not particularly limited, it is preferably not less than 0.001% to develop the action of improving the strength and ductility. The lower limit of the W content is more preferably 0.005%, while the upper limit of the W content is more preferably 0.5%.

B: Not More than 0.005%

B forms precipitates as nitride in steel during and after hot rolling to shape into a rail form and improves the strength and ductility of steel by precipitation strengthening. When the B content exceeds 0.005%, however, martensite is formed, resulting in a decrease in the steel ductility. Thus, when B is included, the B content is preferably not more than 0.005%. Although the lower limit of the B content is not particularly limited, it is preferably not less than 0.001% to develop the action of improving the strength and ductility. Moreover, the upper limit of the B content is more preferably 0.003%.

Ti: Not More than 0.05%

Ti forms precipitates as carbide, nitride, or carbonitride in steel during and after hot rolling to shape into a rail form to improve the strength and ductility of steel by precipitation strengthening. When the Ti content exceeds 0.05%, the coarse carbide, nitride, or carbonitride is produced, resulting in the deterioration of the ductility of steel. Therefore, when Ti is included, the Ti content is preferably not more than 0.05%. Although the lower limit of the Ti content is not particularly limited, it is preferably not less than 0.001% to develop the action of improving the strength and ductility. The lower limit of the Ti content is more preferably 0.005%, while the upper limit of the Ti content is more preferably 0.03%.

Sb: Not More than 0.05%

Sb has a remarkable effect of preventing decarburization of steel during reheating of steel material for a rail conducted in a heating furnace before the hot rolling. However, when the Sb content exceeds 0.05%, the ductility and toughness of steel are adversely affected. Thus, when Sb is included, the Sb content is preferably not more than 0.05%. Although the lower limit of the Sb content is not particularly limited, it is preferably not less than 0.001% to develop the effect of mitigating a decarburized layer. Moreover, the lower limit of the Sb content is more preferably 0.005%, while the upper limit of the Sb content is more preferably 0.03%.

The component composition of the raw steel material to be the material of our rail comprises the above ingredients and the remainder being Fe and inevitable impurities. Also, a rail containing a trace amount of other elements in place of part of Fe in the component composition within the scope of substantially having no meaningful influence on the desired characteristics of the rail. N, 0, and so on are mentioned as the inevitable impurity, where N is acceptable up to 0.008% and 0 is acceptable up to 0.004%.

Moreover, the structure other than pearlite in the microstructure of our rail is not particularly limited. The structure hardly affects the fatigue crack propagation resistance characteristics when the total area ratio thereof is not more than 5% and is thus allowed to be present. Such a structure includes, for example, ferrite, pro-eutectoid cementite, bainite, and martensite.

Next, the method of producing the aforementioned rail will be described below.

The rail can be produced by sequentially subjecting the raw steel material having the above component composition to the following treatments (1) to (3):

-   -   (1) hot rolling     -   (2) primary cooling     -   (3) secondary cooling.         Although the raw steel material used as a rail material can be         produced by an arbitrary method, it is preferable that the raw         steel material be usually produced by casting, particularly         continuous casting.

(1) Hot Rolling

First, the raw steel material is hot-rolled into a rail form. The hot-rolling method is not particularly limited and can use an arbitrary method because the prior austenite grain size of the finally obtained rail can be controlled by controlling a finish rolling temperature in the hot rolling.

Heating Temperature: Not Higher than 1350° C.

The temperature for heating the raw steel material conducted prior to the hot rolling is necessary to be not higher than 1350° C. When the heating temperature exceeds the upper limit, the raw steel material is partly melted by excessive heating and may cause defects in the interior of the rail. Although the lower limit of the heating temperature is not particularly limited, it is preferably not lower than 1150° C. to reduce deformation resistance in the rolling.

Finish Rolling Temperature: Not Lower than 900° C.

When the finish-rolling temperature in the hot rolling is lower than 900° C., the rolling is conducted at a low temperature zone of austenite, resulting in introduced processing strain into austenite crystal grains as well as remarkable elongation of austenite crystal grains. An increase in the austenite grain boundary area causes an increase in the nucleation site of pro-eutectoid cementite, resulting in the deterioration of the fatigue crack propagation resistance characteristics. Therefore, the finish rolling temperature should be not lower than 900° C. Although the upper limit of the finish rolling temperature is not particularly limited, it is preferable to be not higher than 1050° C. because an extremely coarse prior austenite grain size deteriorates the ductility and toughness. The finish rolling temperature means a temperature of a side face of a rail head portion at an entry side of the final rolling mill and can be measured by a radiation thermometer.

(2) Primary Cooling

Average Cooling Rate from 900° C. to 750° C.: 0.4 to 3° C./s

Second, accelerated cooling is conducted. When the average cooling rate of the primary cooling from 900° C. to 750° C., which is a formation temperature region of pro-eutectoid cementite, is less than 0.4° C./s, the amount of pro-eutectoid cementite increases. As a result, the pro-eutectoid cementite tends to cause cracks, which may deteriorate the fatigue damage resistance of the rail. Therefore, the lower limit of the average cooling rate in the primary cooling is preferably 0.4° C./s, more preferably 0.7° C./s. When the average cooling rate in the primary cooling exceeds 3° C./s, a martensite structure may be formed to deteriorate the ductility and fatigue damage resistance. Therefore, the upper limit of the average cooling rate in the primary cooling is preferably 3° C./s, more preferably 2° C./s.

(3) Secondary Cooling

Average Cooling Rate from 750° C. to a Temperature Zone of 400 to 600° C.: 1 to 10° C./s

Secondary cooling is performed after the primary cooling is finished. When the average cooling rate from 750° C. as a start temperature of the secondary cooling to a cooling stop temperature of the secondary cooling in a temperature zone of 400 to 600° C. is less than 1° C./s, the lamellar spacing of the pearlite microstructure is coarsened. This may lower the hardness of the pearlite microstructure to deteriorate the fatigue damage resistance of the rail. In addition, the increase in the cooling time at the low-temperature zone may lower productivity, resulting in an increase in the production cost of the rail. On the other hand, when the average cooling rate in the secondary cooling exceeds 10° C./s, a martensite structure may be produced to deteriorate the ductility and fatigue damage resistance. Therefore, the average cooling rate in the secondary cooling is preferably 1 to 10° C./s. The upper limit of the average cooling rate in the secondary cooling is more preferably 5° C./s.

Each average cooling rate of the primary and secondary cooling is determined using a surface temperature of a side face on the rail head portion and can be measured by a radiation thermometer. The cooling stop temperature in the secondary cooling is a temperature measured on the side face of the rail head portion using the radiation thermometer after the stop of the accelerated cooling (before recuperation).

Examples

The configuration and effects of our rails and methods will be described more specifically in accordance with the following examples. However, this disclosure is not limited by the following examples, and may be modified as appropriate within the scope of conformity with the problems addressed herein, all of which are included in the technical scope of this disclosure.

A rail material was produced by subjecting a raw steel material having a component composition shown in Table 1 to a hot rolling and subsequently to accelerated cooling under conditions shown in Table 2. The accelerated cooling was applied to only a head portion of the rail, which is allowed to cool after the cooling is stopped. The finish rolling temperature in Table 2 is a temperature value of a side surface of the rail head portion measured at an entry side of a final rolling mill by a radiation thermometer. The cooling stop temperature in Table 2 is the temperature value of the side surface layer of the rail head portion measured by the radiation thermometer at a time of cooling stop in the secondary cooling. The cooling rate (° C./s) in each example of the primary cooling and the secondary cooling is a value obtained by converting the temperature change from the start of cooling to the stoppage thereof per unit time (second).

TABLE 1-1 Steel Component composition (mass %) No. C Si Mn P S Cr Other elements X Remarks S01 0.83 0.59 0.48 0.011 0.007 0.91 42133 Inventive Steel S02 0.90 0.35 0.23 0.014 0.011 1.56 62851 Inventive Steel S03 0.85 0.89 0.60 0.009 0.006 0.72 47965 Inventive Steel S04 0.81 0.37 1.79 0.016 0.003 0.25 37467 Inventive Steel S05 0.92 1.19 0.34 0.013 0.005 1.03 71934 Inventive Steel S06 0.84 1.00 0.45 0.016 0.006 0.81 45442 Inventive Steel S07 1.29 0.13 0.76 0.015 0.005 0.22 364626  Inventive Steel S08 0.86 0.58 0.27 0.009 0.004 0.97 50008 Inventive Steel S09 1.00 0.24 0.58 0.034 0.005 0.80 104181  Inventive Steel S10 0.95 0.41 0.34 0.012 0.007 2.01 83425 Inventive Steel S11 1.06 0.70 0.62 0.026 0.003 1.23 143079  Inventive Steel S12 0.89 0.33 0.48 0.011 0.010 0.98 58858 Inventive Steel S13 0.86 0.52 0.73 0.016 0.009 0.67 50004 Inventive Steel S14 0.83 0.11 0.21 0.011 0.010 2.48 42725 Inventive Steel S15 0.85 0.84 0.30 0.010 0.008 1.05 V: 0.05, Nb: 0.018 47938 Inventive Steel S16 0.93 0.60 0.54 0.008 0.004 1.12 Cu: 0.42, Ni: 0.19 74404 Inventive Steel S17 0.87 0.29 0.89 0.014 0.006 0.73 Mo: 0.53 52654 Inventive Steel S18 0.82 0.25 1.35 0.016 0.010 0.58 Al: 0.034, W: 0.26 39502 Inventive Steel S19 0.85 0.47 0.72 0.015 0.009 0.69 B: 0.004, Ti: 0.02 47098 Inventive Steel S20 0.90 1.02 0.36 0.009 0.005 0.45 Sb: 0.04 63141 Inventive Steel S21 0.79 0.36 0.34 0.015 0.005 0.28 31907 Comparative Steel S22 1.31 0.55 0.89 0.018 0.010 0.91 404747  Comparative Steel S23 0.85 0.09 0.25 0.013 0.011 0.72 45750 Comparative Steel S24 1.24 1.23 0.71 0.009 0.009 0.59 312593  Comparative Steel S25 0.85 0.15 0.18 0.020 0.008 0.45 45462 Comparative Steel S26 1.13 0.90 1.81 0.010 0.004 0.50 198892  Comparative Steel S27 0.84 0.28 0.70 0.014 0.005 0.18 43364 Comparative Steel S28 1.08 0.51 1.27 0.012 0.006 2.51 162148  Comparative Steel X = {(10 × [%C]) + ([%Si]/12) + ([%Mn]/24) + ([%Cr]/21)}⁵

Production conditions Test results Average Average Fatigue Number Finish cooling rate cooling rate Prior crack of cycles Heating rolling in primary in secondary Cooling austenite Propagation to failure Test Steel temp. temp. cooling cooling stop temp. grain size: CP = rate * [×10⁻⁸ [×10⁵ No. No. [° C.] [° C.] [° C./sec] [° C./sec] [° C.] R_(A) [μm] X/R_(A) m/cycle] times] Remarks  1 S01 1250 930 1.2 3.4 525 40 1053 4.7 8.25 Example  2 S02 1300 950 1.4 2.6 550 46 1366 5.1 10.25  Example  3 S03 1250 930 1.0 3.1 550 37 1296 5.5 8.50 Example  4 S04 1200 900 0.4 1.2 550 26 1441 7.3 8.75 Example  5 S05 1150 910 0.9 2.5 550 30 2398 7.1 9.50 Example  6 S06 1275 950 1.8 3.6 500 52  874 3.9 8.50 Example  7 S07 1225 1040  3.0 9.8 525 149  2447 7.9 8.25 Example  8 S08 1250 920 2.0 4.0 550 46 1087 4.2 8.50 Example  9 S09 1200 930 0.8 2.9 550 43 2423 6.2 9.25 Example 10 S10 1300 900 1.1 1.8 500 36 2317 5.9 10.75  Example 11 S11 1250 960 0.9 2.6 550 66 2168 5.5 9.50 Example 12 S12 1225 940 1.4 3.7 550 54 1090 4.6 8.50 Example 13 S13 1200 970 0.7 4.1 525 70  714 4.0 8.25 Example 14 S14 1250 950 1.0 4.5 500 61  700 4.2 11.25  Example 15 S15 1300 930 1.5 3.0 550 49  978 4.9 9.25 Example 16 S16 1275 960 2.3 2.8 550 53 1404 5.8 10.50  Example 17 S17 1175 920 0.9 3.6 550 37 1423 4.1 9.00 Example 18 S18 1200 925 1,2 2.3 525 42  941 4.5 8.50 Example 19 S19 1250 940 1.0 4.6 500 49  961 4.3 8.00 Example 20 S20 1300 990 1.6 3.9 550 75  842 5.0 8.25 Example 21 S21 1250 930 1.5 1.1 550 41  778 8.1 6.75 Comparative Example 22 S22 1200 950 0.7 4.8 550 56 7228 9.8 8.50 Comparative Example 23 S23 1200 900 1.0 2.0 500 32 1430 8.5 7.25 Comparative Example 24 S24 1250 920 2.8 4.5 525 35 8931 14.7  8.25 Comparative Example 25 S25 1300 930 0.6 1.8 525 43 1057 8.2 7.75 Comparative Example 26 S26 1225 950 1.9 3.9 550 50 3978 9.2 8.25 Comparative Example 27 S27 1200 970 0.8 1.5 500 69  628 8.3 7.50 Comparative Example 28 S28 1250 940 2.8 4.6 525 53 3059 8.7 8.50 Comparative Example 29 S01 1360 900 1.2 2.9 550 — — — — Comparative Example 30 S10 1250 890 0.7 3.0 500 33 2528 8.2 8.75 Comparative Example *Fatigue crack propagation rate da/dN is a value at a stress intensity factor range ΔK = 20 MPa · m^(1/2). Test No. 29 cannot be evaluated due to partial melting of raw steel material during heating.

An evaluation was conducted on the prior austenite grain size R_(A), fatigue crack propagation resistance characteristics, and fatigue damage resistance of the resulting rail. Each of the evaluations will be described in detail below.

Prior Austenite Grain Size R_(A)

After the finish rolling in hot rolling, a leading portion of the rail was cut and the cut material immediately subjected to a water-cooling treatment. From the thus-obtained water-cooled material, a test specimen for a structure observation was taken out in a longitudinal rolling direction at 5-mm depth from a surface of a rail head portion 1 shown in FIG. 2 . The thus-obtained test specimen was subjected to mirror polishing and then etching with γ-grains, and a section thereof was observed with an optical microscope of 200 magnification. The prior austenite grain size R_(A) was evaluated by measuring grain sizes of 400 or more grains by a trace operation using image analysis software and calculating an average value thereof

Fatigue Crack Propagation Resistance Characteristics

Test specimens for fatigue crack propagation were taken out from two positions of the rail head portion and gauge corner (GC) portion shown in FIG. 3 to conduct a fatigue crack propagation test. FIG. 4 is a schematic view showing one example of the test specimen, where FIG. 4(a) is a front view, FIG. 4(b) is a side view, and FIG. 4(c) is an enlarged front view of a notch portion. In FIG. 4 , the test specimen has a plate shape with, for example, a width W=20 mm, a height H=100 mm, and a thickness B=5 mm, where a notch portion is formed in an end part of a central portion H/2 of the height H. The notch portion has a length L=2 mm and a width C=0.2 mm where an end part of the notch portion is formed at a radius of curvature R=0.1 mm. The stress ratio (R ratio=minimum stress/maximum stress) was 0.1, and the fatigue crack propagation resistance was evaluated by measuring a fatigue crack propagation rate da/dN (m/cycle) at a stress intensity factor range ΔK=20 MPa·m^(1/2). When the value of da/dN is not more than 8.0×10⁻⁸, the material was evaluated to have characteristics of suppressing fatigue crack propagation.

Fatigue Damage Resistance

Although it is most desirable to evaluate the fatigue damage resistance by actually laying rails, this requires a long time for testing. Therefore, a Nishihara-type wear specimen, which can evaluate fatigue damage resistance in a short time, was used. In the test, fatigue damage resistance was evaluated by a comparative test simulating actual contact conditions between a rail and a wheel. Concretely, the test was conducted by taking out a Nishihara-type wear specimen 17 having a diameter of 30 mm from the rail head portion 1 provided that a contact face is a curved face having a radius of curvature of R=15 mm and rotating the specimen 17 in contact with a wheel specimen 18 as shown in FIG. 5 . The wheel specimen 18 was prepared by taking out a rod having a diameter of 32 mm from a head portion of a regular rail described in JIS E1101:2012, subjecting the rod to a heat treatment to have a Vickers hardness (load: 98 N) of Hv 390 and a tempered martensite structure, and working into a cylinder shape having a diameter of 30 mm. The Nishihara-type wear specimen 17 was taken out from a fatigue damage resistance specimen collection area 14 on the surface of the rail head 1 as shown in FIG. 6 . Arrows in FIG. 5(a) indicate the rotating directions of the Nishihara-type wear specimen 17 and the wheel specimen 18, respectively. Under such a test environment of oil-lubrication conditions as a contact pressure: 1.8 GPa, a slip ratio: −20%, and a revolution rate: 600 rpm (750 rpm in the wheel specimen), the surface of the specimen was observed every 2.5×10⁴ times, and the revolution number when a crack of not less than 0.5 mm was caused was defined as the service life for fatigue damage. When the value of the revolution number is not less than 8×10⁵ times, the specimen is judged to have a fatigue damage resistance.

The test results are also shown in Table 2. In the test results (Test Nos. 1 to 20, . . . in Table 2) of the rails that are made of acceptable steels satisfying our component composition and CP of not more than 2500 and produced by the production method within our ranges (heating temperature, finish rolling temperature), all the fatigue crack propagation rate da/dN (m/cycle) at ΔK=20 MPa·m^(1/2) satisfy not more than 8.0×10⁻⁸. Also, Test Nos. 1 to 20, in which the primary cooling and secondary cooling conditions are in preferable ranges, satisfy both the fatigue crack propagation rate da/dN (m/cycle) of not more than 8.0×10⁻⁸ and the fatigue damage service life of not less than 8×10⁵ times. In Comparative Examples (Test Nos. 21 to 28, and 30 in Table 2) where the component composition of the rail material does not satisfy our conditions or our production method is not adopted, CP exceeds 2500 and the fatigue crack propagation rate da/dN (m/cycle) exceeds 8.0×10⁻⁸ or the fatigue damage service life is less than 8×10⁵ times. In Test No. 29, the heating temperature was too high so that a part of the raw steel material was melted during the heating. As a result, the steel material could not be subjected to the rolling due to possible breakage during rolling, and hence the characteristics could not be evaluated.

INDUSTRIAL APPLICABILITY

Our rail and production method thereof can stably produce a fatigue damage-resistant rail having an excellent fatigue crack propagation resistance characteristics, which contributes to increasing the service life of the rail for a high-axle load railway and preventing railroad accidents, thereby bringing about industrially beneficial effects. 

1-5. (canceled)
 6. A rail having excellent fatigue crack propagation resistance characteristics, wherein the rail has a component composition comprising C: 0.80 to 1.30 mass %, Si: 0.10 to 1.20 mass %, Mn: 0.20 to 1.80 mass %, P: not more than 0.035 mass %, S: 0.0005 to 0.012 mass %, Cr: 0.20 to 2.50 mass % and the remainder being Fe and inevitable impurities and satisfying CP represented by equation (1) being not more than 2500: CP=X/R _(A)  (1) and X={(10×[% C])+([% Si]/12)+([% Mn]/24)+([% Cr]/21)}⁵  (2), where [% Y] is content of an element Y (mass %), and R_(A) is a prior austenite grain size (μm).
 7. The rail according to claim 6, wherein the component composition further contains at least one selected from V: not more than 0.30 mass %, Cu: not more than 1.0 mass %, Ni: not more than 1.0 mass %, Nb: not more than 0.05 mass %, and Mo: not more than 2.0 mass %.
 8. The rail according to claim 6, wherein the component composition further contains at least one selected from Al: not more than 0.07 mass %, W: not more than 1.0 mass %, B: not more than 0.005 mass %, Ti: not more than 0.05 mass %, and Sb: not more than 0.05 mass %.
 9. A method of producing a rail having excellent fatigue crack propagation resistance characteristics, wherein a raw steel material having a component composition according to claim 6 is heated to not higher than 1350° C. and hot-rolled such that a finish temperature is not lower than 900° C.
 10. The method according to claim 9, wherein, after the hot rolling, accelerated cooling is conducted at a cooling rate of 0.4 to 3° C./s from 900° C. to 750° C. and at a cooling rate of 1 to 10° C./s from 750° C. to a cooling stop temperature of 400 to 600° C.
 11. The rail according to claim 7, wherein the component composition of the raw steel material further contains at least one selected from Al: not more than 0.07 mass %, W: not more than 1.0 mass %, B: not more than 0.005 mass %, Ti: not more than 0.05 mass %, and Sb: not more than 0.05 mass %.
 12. The method according to claim 9, wherein the component composition of the raw steel material further contains at least one selected from V: not more than 0.30 mass %, Cu: not more than 1.0 mass %, Ni: not more than 1.0 mass %, Nb: not more than 0.05 mass %, and Mo: not more than 2.0 mass %.
 13. The method according to claim 9, wherein the component composition of the raw steel material further contains at least one selected from Al: not more than 0.07 mass %, W: not more than 1.0 mass %, B: not more than 0.005 mass %, Ti: not more than 0.05 mass %, and Sb: not more than 0.05 mass %.
 14. The method according to claim 12, wherein the component composition of the raw steel material further contains at least one selected from Al: not more than 0.07 mass %, W: not more than 1.0 mass %, B: not more than 0.005 mass %, Ti: not more than 0.05 mass %, and Sb: not more than 0.05 mass %.
 15. The method according to claim 12, wherein, after the hot rolling, accelerated cooling is conducted at a cooling rate of 0.4 to 3° C./s from 900° C. to 750° C. and at a cooling rate of 1 to 10° C./s from 750° C. to a cooling stop temperature of 400 to 600° C.
 16. The method according to claim 13, wherein, after the hot rolling, accelerated cooling is conducted at a cooling rate of 0.4 to 3° C./s from 900° C. to 750° C. and at a cooling rate of 1 to 10° C./s from 750° C. to a cooling stop temperature of 400 to 600° C.
 17. The method according to claim 14, wherein, after the hot rolling, accelerated cooling is conducted at a cooling rate of 0.4 to 3° C./s from 900° C. to 750° C. and at a cooling rate of 1 to 10° C./s from 750° C. to a cooling stop temperature of 400 to 600° C. 